The use of spectrophotometry has been applied to a wide range of materials to determine properties by measuring the adsorbance of radiation through gas, liquid, or solid samples. In the most common arrangements, radiation passes from an emitter, through an entrance window, a sample and an exit window on its way to a detector. Specifically, fluid samples may be withdrawn for flow through a dedicated sample cell or a probe may introduce one or both of the windows into a sample stream or reservoir. The probes and flow cells typically include a lens to collimate or focus radiation through the window in a desired pattern. The direction of radiation transmission and detection may be parallel to the flow of a sample through the cell or the radiation and sample may have a crossflow arrangement. Parallel and cross-flow arrangements for cells are disclosed in U.S. Pat. Nos. 4,192,614 and 5,078,493; the contents of which are hereby incorporated by reference. Fiber optic cables transmit radiation from a radiation source to the sample point and from the sample point to a radiation detector. The operation of a spectrophotometer apparatus and its method of use are fully disclosed in U.S. Pat. No. 4,786,171; the contents of which are hereby incorporated by reference. U.S. Pat. No. 5,140,661 shows details of fiber optic cable and connectors for use therewith.
Probes typically comprise an opposing pair of radiation transparent windows that allow radiation to pass from a radiation emitter, through the sample and on to a detector. The insertion of these probes into process streams exposes them to temperature and pressure extremes as well as to other environmental conditions such as humidity, vibration, and corrosively that are associated with the particular process that uses them. Thermal expansion of different probe components poses a substantial problem. The probe bodies are typically made of stainless steel or other higher metallurgy which has a coefficient of expansion on the order of 10 times higher than coefficient of expansion of the silica material that forms most optical fibers. Probe arrangements routinely insert optical fibers, with or without sheathing, into stainless steel tubes which extend into a probe body. At probe operating temperatures of about 300.degree. C., the differential expansion exerts tension on the optical fibers and imposes significant stress on the fiber bonds at opposite ends of the tubes. The typical small internal diameter of the tubes that contain the fiber optic cables, on the order of 3/16.sup.th of an inch or less, severely restricts the packing of excess fiber into the tube to accommodate the differential expansion between the fiber and the tube. In most cases the packing of excess fiber only amounts to a few thousandths of an inch. Failure of these bonds would result in loss of optical throughput or efficiency, possible appearance of spectral or temporal etalon fringes (Edser-Butler fringes), fractures in the fiber, and excess sensitivity to vibration.
In addition to the direct stresses imposed by thermal expansion, taut fibers vibrate at much higher frequency than loose fibers, often producing frequencies and amplitudes that interfere with the radiation detection and sometimes cause fiber failure. Dampening of the taut fibers by the use of filler material within the tubes often fails to provide an adequate solution due to deterioration of the filler material at high operating temperatures.
It is known in the prior art to use fiberglass sleeving to protect fiber optic cables from thermally induced degradation at high temperature. U.S. Pat. No. 4,896,940 teaches the covering of single optical fibers or fiber optic cables with several protective layers to prevent damage from excessive temperatures and tension on the cables. According to the '940patent, loose fiberglass sleeving may comprise a thermally protective layer . The arrangement of the '940 patent also places Teflon wrapping on the outside of the protective layer that covers the optical fiber. In this arrangement, a protective layer separates the Teflon wrapping from the optical fiber.
Accordingly, there is need for a probe with improved capacity to withstand differential expansion of its components.
There is also need for a probe that has improved resistance to damage imposed by vibration of the optical fiber within the probe and induced spectral or temporal noise resulting from vibration.
Accordingly, it is an object of this invention to provide a probe having an increased ability to accommodate differential thermal expansion between the materials of probe construction.
It is a further object of this invention to provide a probe that will minimize or eliminate damage from the vibration of optical fibers and spectral or temporal noise associated with vibration of optical fibers within the probe.